1. Field of the Invention
The present invention relates to an alumina sintered compact obtained by firing an alumina powder, and a method of producing the same, and an article for heat treatment using the alumina sintered compact.
2. Description of the Related Art
Alumina sintered compacts have satisfactory corrosion resistance and heat resistance and are generally used as firing tool materials such as boxes and cases or as tubes for heat treatment of items such as atmospheric gas supply tubes. This type of alumina sintered compacts includes a fine-grain-containing alumina sintered compact obtained by sintering a fine alumina powder, as described in Japanese Unexamined Patent Application Publication No. 4-231370, and an oriented alumina sintered compact obtained by firing a plate fine alumina powder to thereby the subject crystal grains to grain growth in layers, as described in Japanese Unexamined Patent Application Publication No. 7-315915.
However, when a ceramic article containing PbO, for example, is fired using a firing tool composed of the fine-grain-containing alumina sintered compact, the sintered compact is deteriorated due to the PbO, and the life of the firing tool or tube for heat treatment is shortened in some cases. This is probably because the crystal grains of the sintered compact are dense compacts of fine grains and therefore have a lot of grain boundaries and a large amount of PbO is absorbed by the grain boundaries. The strength and creep characteristics of the sintered compact are thus deteriorated to thereby invite warpage and deformation.
On the contrary, an oriented alumina sintered compact absorbs PbO less than the fine-grain-containing alumina sintered compact, as the crystal grains are formed by grain growth. However, repeated firing procedures of the oriented alumina sintered compact permit PbO to accumulate on a surface area of the sintered compact to thereby invite the following problems:
(1) The PbO absorption by the alumina sintered compact gradually decreases and the atmosphere varies every time the sintered compact is used.
(2) The sintered compact has a large amount of PbO on its surface, which PbO is liable to react with the ceramic article and adversely affects the quality of the ceramic article in some cases.
(3) PbO accumulated on the surface area expands and contracts in repeated heat cycles, and the crystal grains on the surface area may therefore peel or drop off to thereby invite the formation of a rough surface or depressions on the surface of the alumina sintered compact.
Under these circumstances, accordingly, an object of the present invention is to provide an alumina sintered compact that can prevent the accumulation of PbO on its surface while controlling the absorption of PbO, an article for heat treatment using the alumina sintered compact, and a method of producing the alumina sintered compact.
The present inventors made investigations on the crystal structure of an alumina sintered compact from the viewpoint of preventing the accumulation of PbO while controlling the absorption of, for example, PbO. They found that the corrosion resistance to, for example, PbO is improved when the surface area of the alumina sintered compact is composed of a layer of alumina crystal grains which have a large mean grain size and are obtained by grain growth, and that the accumulation of, for example, PbO on the surface area can be prevented by allowing the inside of the sintered compact to absorb and disperse PbO or the like attached to the surface area. The present invention has been accomplished based on these findings.
Specifically, the present invention provides, in an aspect, an alumina sintered compact which includes at least two layers including an internal layer and a surface layer formed on the surface of the internal layer. In the alumina sintered compact, the internal layer includes alumina crystal grains formed by sintering of a fine alumina powder, and the surface layer includes alumina crystal grains formed by grain growth of a fine alumina powder and having a mean grain size larger than that of the alumina crystal grains of the internal layer.
The alumina sintered compact of the present invention may be obtained, for example, by the following procedure. A high purity granular fine alumina powder which is readily sintered is admixed with a high purity plate fine alumina powder as a seed crystal and the resulting mixture is cast and molded using a mold to thereby yield a green compact. In the green compact, the plate fine alumina powder is oriented in the vicinity of the surface in layer parallel to the surface and the granular fine alumina powder is compacted inside the green compact. The green compact is then fired at a temperature of preferably about 1550xc2x0 C. to 1750xc2x0 C. to thereby form a surface layer and an internal layer, which surface layer is mainly composed of alumina crystal grains formed by grain growth of the plate fine alumina powder on the surface, and which internal layer is mainly composed of alumina crystal grains formed by compact bonding of the granular fine alumina powder. Additionally, the grain sizes of the alumina crystal grains on the surface area and the thickness of the surface layer can be controlled by changing the ratio of the plate fine alumina powder to the granular fine alumina powder.
In the alumina sintered compact of the present invention, the alumina crystal grains constituting the surface layer preferably have a mean grain size between about 50 xcexcm and 1000 xcexcm and the alumina crystal grains constituting the internal layer preferably have a mean grain size between about 1 xcexcm and 50 xcexcm.
The term xe2x80x9cgrain sizexe2x80x9d of alumina crystal grains as used herein means the average of the maximum diameter and the minimum diameter of each crystal grain in a cross section perpendicular to the surface layer of the resulting sintered compact.
The mean grain size of the surface layer preferably falls in a range from about 50 to 1000 xcexcm. This is because satisfactory corrosion resistance may not be obtained if the mean grain size is less than about 50 xcexcm, and strength may be deteriorated due to abnormal grain growth if it exceeds about 1000 xcexcm. The mean grain size of the internal layer preferably falls in a range from about 1 to 50 xcexcm. This is because an alumina crystal grain having a mean grain size of less than about 1 xcexcm may not be significantly prepared and PbO or the like may not be effectively absorbed and dispersed if it exceeds about 50 xcexcm.
In the invented alumina sintered compact, the surface layer preferably has a thickness between about 100 xcexcm and 2000 xcexcm.
If the thickness of the surface layer is less than about 100 xcexcm, sufficient corrosion resistance may not be obtained, and if it exceeds about 2000 xcexcm, PbO or the like may not be sufficiently absorbed by and dispersed into the internal layer to thereby invite the accumulation of PbO or the like on the surface area.
The aspect ratio of the alumina crystals of the surface layer in the invented alumina sintered compact is preferably between about 1:3 and 1:10. The term xe2x80x9caspect ratioxe2x80x9d as used herein means the ratio of the length in thickness direction to the length in width direction of a layered alumina crystal grain.
The preferred aspect ratio is specified in a range from about 1:3 to 1:10. If the aspect ratio is less than about 1:3, satisfactory corrosion resistance may not be obtained, and if it exceeds about 1:10, strength may be deteriorated due to abnormal grain growth. Additionally, alumina crystal grains each having an aspect ratio within the above range, excluding crystal grains formed by abnormal grain growth, preferably occupy about 90% or more of the total alumina crystal grains.
In another aspect, the present invention provides an article for heat treatment which includes the invented alumina sintered compact.
Such articles for heat treatment according to the present invention include, for example, boxes, cases and other firing tool materials, refractory materials, atmospheric gas supply tubes, radiant tubes and other tubes for heat treatment.
In addition and advantageously, the present invention provides a method of producing an alumina sintered compact, which includes the steps of mixing fine alumina powder A and fine alumina powder B, which fine alumina powder A has a mean grain size between about 0.4 xcexcm and 3.0 xcexcm, contains plate Al2O3 grains and has a purity of 99% or more, and which fine alumina powder B has a mean grain size between about 0.3 xcexcm and 2.0 xcexcm and having a purity of about 99% or more; casting and molding the resulting mixture using a mold to yield a green compact; drying the green compact; and firing the dried green compact at a temperature between about 1550xc2x0 C. and 1750xc2x0 C.
The mean grain size of fine alumina powder A is specified in a range from about 0.4 to 3.0 xcexcm. If the mean grain size is less than about 0.4 xcexcm, the grains do not satisfactorily undergo grain growth, and if it exceeds about 3.0 xcexcm, sinterability may be deteriorated. The mean grain size of fine alumina powder B is specified in a range from about 0.3 to 2.0 xcexcm. If the mean grain size is less than about 0.3 xcexcm, sintered density may be decreased, and if it exceeds about 2.0 xcexcm, sinterability may be deteriorated.
The firing temperature is specified between about 1550xc2x0 C. and 1750xc2x0 C. because the grains may not sufficiently undergo grain growth if the firing temperature is lower than about 1550xc2x0 C. and the grains may abnormally grow if it exceeds about 1750xc2x0 C.
The weight ratio of fine alumina powder A to fine alumina powder B is preferably between about 1:9 and 4:6 inclusive. If the weight ratio exceeds about 4:6, the surface layer may have an excessively large thickness to thereby invite the grain growth of the overall alumina sintered compact in some cases. If the weight ratio is less than about 1:4, the surface layer may not have a sufficient thickness.
The invented alumina sintered compact can prevent the absorption of, for example, PbO by action of the surface layer. This configuration can prevent the strength and creep characteristics of the sintered compact from deterioration and can prolong its life when it is used as, for example, a firing tool material or a tube for heat treatment.
Even when PbO or the like accumulates on the surface area due to repeated firing procedures, the internal layer can absorb and disperse the accumulated matter to thereby prevent PbO or the like from accumulating in the vicinity of the surface area. This configuration can avoid adverse effects on the quality of a ceramic article due to a reaction between PbO or the like and the ceramic article, can prevent peeling and dropping off of crystal grains due to heat cycling and can improve the reliability of the quality. Additionally, the internal layer absorbs and disperses PbO or the like to thereby level the absorption of PbO or the like to thereby uniformize the atmosphere every time when the sintered compact is used. The reliability of the quality can also be improved also by this configuration.
A preferred alumina sintered compact of the present invention including a surface layer having a mean grain size between about 50 xcexcm and 1000 xcexcm and an internal layer with a mean grain size between about 1 xcexcm and 50 xcexcm can enhance the absorption and dispersion of PbO or the like in the internal layer while improving the corrosion resistance of the surface layer.
Another preferred alumina sintered compact of the present invention comprising a surface layer having a thickness between about 100 xcexcm and 2000 xcexcm can ensure the internal layer to absorb and disperse PbO or the like and can improve the corrosion resistance of the surface layer.
The invented alumina sintered compact as another preferred embodiment, in which the aspect ratio of the surface layer is between about 1:3 and 1:10, can avoid the deterioration of strength due to abnormal grain growth and can ensure the required two-layer structure and can improve the corrosion resistance to, for example, PbO.
When the invented alumina sintered compact is used as an article for heat treatment such as a firing tool material, refractory material or a tube for heat treatment, the corrosion resistance in repeated firing procedures of a ceramic article containing PbO or the like can be improved to thereby prolong the life of the article for heat treatment.
The invented method of producing an alumina sintered compact, in which plate alumina powder A and granular alumina powder B are subjected to casting-molding using a mold, can repetitively produce the invented alumina sintered compact including two layers composed of a surface layer formed by grain growth and an internal layer formed by fine dense grains.